System of modulating carrier currents



oct. 16, 192s. i 1,688,292

A. WEAVER SYSTEMl OF. MODULATING CARRIER GURENTS ATTORNEY Filed March v 1925 2v sheets-sheet 1 A. WEAVER SYSTEM OF MODULATING CARRIER CURRENTS 2 Sheets-Sheet 2 Filed March '7, 1925 lNvENToR /YQ Wamel' i ATroRNEY Patented Oct. 16, 1928..

UNITED STATES PATENT OFFICE.

ALLAN WEAVER, OF BROOKLYN, NEW YORK, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION F NEW- YORK.

SYSTEM OF MODULATING CARRIER CURRENTS.

Application led March 7, 1925. vSerial No. 13,925.

An object of my invention is to provide for the neutralization or balancing out of the effect of the capacity of a light controlled photoelectric cell, or other light sensitive device, introduced into the circuit of the cell bythe cell itself. This object of my invention and others will becomereadily apparent on consideration of a limited number of specific embodiments of the invention which I havechosen to illustrate in the accompanying drawings, and which I now proceed to describe. It will be understood that this description refers to these lparticular examples of the invention and that the invention will be deined in the appended claims.

Referrin to the drawings, Figure 1 is a diagram sowing one embodiment of my invention; Fig. 2 is a characteristic curve for the relation of output current to light intensity; Fig. 3 is a diagram of a modification .as compared with Fig. l; Fig. 4 is the corresponding characteristic for Fig. 3; Fior.l

5 is a characteristic for a photoelcctric cell to which reference will be made in explaining the adjustment of my system, and Fig. 6 is another modification of Fi l.

Referring to Fig. 1, light trom the source L is'focused by a lens on the opening in the 3.0 screen S. Behind the screen S is a picture film F wrapped on a glass cylinder which is rotated on its Iaxis and traversed relatively slowly along its axis by a 'screw threaded mounting for its axial shaft. Accordingly,

the spot on the film F, underlying the open- .ing in the screen S travels a helical course and covers the entire The photoelectric cell; PC receives the light through the opening in the screen S 40 as determined in intensity., byV the degree of shade in the underlying lifportion of: the film F. p

A carrier current of say 1,000 cycles per second is applied through the transformer T1 across the extreme terminals ofthe series resistance R, and R2. These resistances constitute two of the four arms of a Wheatstone bridge; the adjustable capacity C2 is in a 4admittance of about third arm and the photoelectric cell PC is in the fourth arm and its electrostatic capacity is represented as C1.

When the relation obtains, the system constitutes a balanced Whcatstone bridge and the carrier current source will put no electromotive force across the terminals of the resistance R3, assuming that no light is falling on the photoelectric 6 cell PC so that it corresponds accurately to a condenser of capacit C1.

But when light fa ls from the source L on the cell PC a current is established with lthree-electrode. vacuum tube amplier, A.

The output from this tube is further amplified and is put on the line for transmission.

The importance of neutralizing the capacity in the photoelectric cell will ibe appreciated from the following rough computation of an assumed example.' A reasonable value to assume for the capacity of the cell mayv be about 3 l012 microarads at 1,000 cycles per second. This corresponds to anv impedance of about 50 megohms or an 5` Under normal 'operating conditions the maximum steady current usually obtained from the cell due to light falling upon it, corresponds to an impedance throughthe cell of about 300 megohms (or even greater), or an admittance of about goywr less)h. Accordingly, it will be seen that the alternatingcurrent 50 within the range yis varied by the incidence of light only From this it is seen that the impedance, or reciprocally, the admittance of the cell is not greatly changed relatively by the light falling uponsit, consequently the car rier current would not be as effectively modulated as when the capacity of the cell is neutralized.

The battery B1 in Fig. l is employed to bring the operating point on the most favorable portion of the characteristic, which Will be its steepest part. This is illustrated in Fig. 5, which shows a typical characteristic curve of a photoelectric cell. A good adjustment for the battery B1 Will be to the value represented by the abscissa 0a.

Fig. 2 gives a characteristic of output alternating current as a function of light intensity on the cell. This characteristic starts at zero current (When the bridge is in balance) for zero light and increases approximately linearly as the light on the cell is increased.

Referring to Fig. 3, this shows a system arranged to give a negatively sloped characteristic (that is the output decreases as c the light increases) as shown in Fig. 4. If

the receiving apparatus to go with Fig. l

- produces a positive, then that same receiving apparatus Without adjustment, when associated to receive from Fig. 3, Will produce a negative, and vice versa. It will be seen that in Fig. 3 the carrier current is applied as in Fig. 1, and also by a branch circuit through the potentiometer P1 and the transformer T2 the carrier voltage is applied in phase opposition to the output from the bridge. Accordingly, when the electromotive force applied through the potentiometer l?1 and through the output from thel vbridge are equal, the input on the amplifier A will be zero, whereas, Without the auxiliaryE voltage through the potentiometer l?1 the Youtput at this time would be a maximum.

. By throwing the switches Q1 and Q2 the ap- Y Y opposition becomes exact.

paratus can be changed; with one adjustthe other adjustmentto'send from negative to positive or positive to negative.

The voltage at the secondary of the trans. former T2 will notbe exactly 180 out of phase with the voltage coming from the ridge, so an adjustable condenser C3 is placed in parallel with the secondary of the transformer to adjust the phase until the' The system of Fig. 3 should be adjusted as follows;

' ments.

1. Set the potentiometer P1 at zero, that is, so that it gives zeroI voltage at'the primary of the transformerTg. Adjust condenser C2 until the output from the bridge, as indicated by a thermocouple, placed in the output of the amplifier temporarily for measuring, is zero with no light shining on the cell. y

2. Allow maximum light to shine on the cell and adjust either battery B1 or the in? tensity of the carricrsource until an output current is obtained Which is deemed to be a suitable maximum.

3. Adjust the potentiometer P1 until the output current is reduced to a minimum.

4.,Adjust the condenser C3 'until the output current is further reduced to zero. This adjustment Will stand unless the frequency of the carrier is changed.

If the adjustment has been made as outlined above, the apparatus Will give a characteristic of output current as a function of light intensity corresponding to Fig. 4.

It Will be seen that by my invention I have provided for modulating the carrier current directly in a photoelectric| cell; this may be effected by light determined by the degree of shade of successive picture ele- Accordingly, the output ofv the 'p hotoelectric cell is a modulated picture transmitting current which needs only to be amplified to be transmitted. An advantage of my system is that it gives a satisfactory approximation to a linear characteristic for` the output current intensity as a function 100' of the degree of light on the cell.

In Fig. 6 I have shown another Way of neutralizingthe electrostatic capacity of the cell. In this case a branch circuit from the carrier source goes to 'the transformer T2, 105 and adjustment is made at C4 and D so that When -no light falls on the cell PC, its capacity is neutralized and there is no electromotive force on the grid of theaudion A.

I claim: `H0

l. In combination, a photoelectric cell, means to apply -a carrier current electrometive force to its terminals, ,means to apply to the cell a light` varyingy in a determinate manner Whereb the current through the cell is a correspon ing modulated carrier cur-` rent, an output circuit, and adjustable means in said output circuit/,to make the magnitude of the output vary/directly or inversely as the light as desired.

2. In combination, a vphotoelectric cell,l

means to apply a carrierl current electromotive force/"to its terminals, an associated output circuit, and means to apply a counter eleetromotfive force in said output circuitwherebyy the out ut current will decrease with increased lig t on the said cell.

3. In combination, a photoelectricV cell, means to apply a carrier current electromotive force to its terminals, means to neutralize the capacity of the cell, and means to apply to the cell a li ht varying in a determinate manner w ereby the current through .the cell is a corresponding modu lated carrier current.

4. In` combination, a y photoelectric cell,

means to apply an alternating electrometiveforce to its terminals, an output circuit associated therewith, means to neutralize the electrostatic capacity of the cell, and means to apply to the cell alight varying in a determinate manner whereby the current in said output circuit is a corresponding modu-4 lated 'carrier current independent of said capacity.

5. The method of generating a picture transmitting modulated carrier current which consists in applying the carrier electromotive force to the terminals of a photoelectric cell, neutralizing the capacity of said cell, and illuminating the cell in degree according to the degree of shade of'successive elements of area of the picture to be transmitted, whereby the current through the cell is al modulated carrier current suitable4 for transmission ot the picture.M

6. In combination, a photoelectric cell,

means to apply a carrier current electromotive torce across its terminals, means to apply hght to the.` cell graduated according to the shade of successive elements of a lpicture representation, whereby the current through the cell -is a modulated carrier current suitable for transmission of the picture, and ad- ]iistable means adapted at one adjustment to determine this current for sending from a positive representation to reproduce a posit1ve or from a negatwe to reproduce a negative, a-nd at another adjustment to determine the current for sendlng from a positive to reproduce a negative or from a negative to re- -produce a positive.

7. In combination, a VVheatstone bridge comprising a source of carrier current, a photoelectric cell in one arm, a condenser in another arm and impedance elements in the remaining arms whereby. the electrostatic capacity of the cell is neutralized and the bridge is in balance with no light falling on the cell, and means to apply light varying in a determinate manner to the cell so that the:

" unbalance current from the bridge will bea modulated carrier current. f

8. In an electro-optical vimage producing system, la light sensitive device, means for producing an. alternating current in a circuit including said light sensitive device,

lmeans for actuating said light sensitive device in accordance with thel shades of successive elemental areas of a picture or object an image of which is to be produced, thereby modulating lsaid alternating current in accordance wlth the shades of said elemental areas, and means forneutrahzino that component of the impedance of saidtlight sensi-v pressed thereon is varied, thereby increasin the effectiveness of the modulating produced by said light sensitive-device.

`9. In an electro-optical image producing system, a light sensitive device, means for producing an alterntaing current in a circuit including said light sensitive device, means for actuating said light sensitive device in accordance with the shades of successiveelemental arcas of a picture or object an image of which is to be produced, thereby modulating said alternating current in accordance with the shades ofI said elemental areas, and means for neutralizing the capacity of said light sensitive device to increase the effectiveness of the modulating produced thereby.

10.` In an electro-optical image producing system, alight sensitive device, means for producing an alternating current in a cir cuit including said light sensitive device, means for actuating sald light sensitive device in accordance with the shades of successive elemental areas of a picture or object the image of which is to be produced, thereby modulating said alternating current in acofthe impedance of said light sensitive device which remains substantially constant when the intensityof the light impressed thereon is varied, thereby increasing the eiectiveness of the modulating produced by said light sensitive device, and means for transmitting said modulated current to a distant station for controlling the production of an image of said pictureor object.

11. The method of modulating a carrierv current in a determinate sequence which comprises causing a carrier current to flow through a light sensitive device, var ing the illumination-of said light sensitive evi-cein accordance with said determinate sequence for modulating said carrier-current accordingly, and neutralizing that component of the'impedance of said light sensitive; device which remains substantially constant when the intensity of said illumination is varied, thereby increasing the eiliciency with which said carrier current is modulated.

12. Themethod of modulating a carrier current in a determinate sequence which` comprises causing a carrier current to flow through a light sensitive device, varying the illumination of'said light sensitive device in accordance with' said determinate sequence, and neutralizing the capacity of said light sensitive device to increase the modulation efficiency thereof'.

In testimony whereof, I-have signed my name to this specification this 6th 'day of March, 1925. Y

ALLAN WEAVER. 

